This invention relates to optical data storage systems, and more particularly to a means for organizing data on an optical disk or platter of such a system in order to provide efficient handling, storing, detection, and processing of said data.
Over the past two decades or so, there have been two major trends in the data processing industry that have worked together to revolutionize the way that information is gathered, stored, and interpreted. The first trend has been the expansion of technological sophistication, as exemplified by the microcomputer chip. That is, computing power, which once required roomsful of equipment and kilowatts of electrical power to operate, can now be found in very small silicon chips. The second trend has been the cost of purchasing such computing power. Particularly in the area of memory--as costs have dropped and capacities have increased--there has been an inevitable rush to take advantage of the new-found memory space and fill it with information. In this respect, the demand for more memory and storage space has always seemed to outstretch the available supply of such memory space.
Unfortunately, for users with exceptionally large data storage needs, the magnetic-based storage peripheral devices adapted for use with high performance computers (i.e., magnetic tape and disk drives) have not been able to fill the need for more storage space. Traditionally, the need for more storage space in such large data storage systems has been addressed by merely adding additional magnetic disk drives and/or magnetic tape drives. This has been costly both in terms of expense (purchase/lease price plus maintenance costs) and floor space. Moreover, even though there have been some significant strides in recent years with respect to increasing the data storage capacity of these magnetic-based storage devices, the theoretical design limits of such systems are rapidly being approached. Hence, merely adding more magnetic disk or tape drives is no longer viewed as a practical alternative to the ever increasing need for storing more and more information. It is therefore apparent that a new type of data storage system is needed in order to handle the large amounts of data that information users need to store.
Optical technology--that is, the technology of using a laser beam to burn or otherwise mark very small holes on a suitable medium in a pattern representative of the data to be stored, which pattern can subsequently be read by monitoring a laser beam directed through or reflected off of the previously recorded marks--has been available in laboratories for some time. Unfortunately, however, such laboratory technology has not provided a cost effective alternative for use in data storage products. This is because the optical components have tended to occupy entire rooms and the power associated with operating the laser and associated components has been enormous. Further, such laboratory systems are not easily interfaced with existing high performance computer systems. That is, the techniques used to format and input the data have been totally incompatible with more conventional formatting and data processing techniques used in the magnetic-based storage systems. Moreover, the few optical storage systems that have been commercially introduced in the last few years have primarily related to the storing of video signals (image storing devices) as opposed to the storing of digital information. Further, the few digital optical storage devices that do not exist do represent a viable alternative or supplement to the existing peripheral magnetic-based storage devices for the user of large data bases of information.
A continuing problem that has existed with whatever type of data storage system is used is the problem of minimizing the errors that occur during read or write. The number of errors that occur in such a system is typically measured by a parameter referred to as the "bit error rate." This parameter is typically expressed as a number indicating the number of good bits of digital data that can be obtained for every bad bit of data that occurs. Thus, a bit error rate of 100,000 (10E+5) indicates that 100,000 bits of data can be read or played back before a bad or incorrect bit of data will be encountered. In order to provide a viable data storage system, bit error rates in excess of 10E+12 are generally required.
Numerous Error Correction codes (ECC) and similar error correcting schemes are shown in the art in order to improve the bit error rate of data processing systems. The very existence of such ECC schemes evidences the continuing and recurring problem of reducing errors that are introduced into such processing systems. Errors can principally originate from one of three sources: (1) in the write channel (i.e., the data is written incorrectly); (2) in the read channel (i.e., the data is read incorrectly); or (3) in the storage medium (i.e., even though the data is initially written correctly, the storage medium may change with time so as to alter the data to make it incorrect). Of these three potential sources of error, most known ECC schemes for use with peripheral data storage systems are directed only to correcting errors that occur in the read channel. Sources of error introduced by aging media, item (3) above, are minimized in magnetic-based storage devices by merely re-writing the data after a prescribed period of time. (This technique is commonly referred to as "refreshing.") That is, the old data is read, stored in a buffer memory, the media is erased, and new data is then written on the media in place of the old data. This refreshing technique is, of course, only available when erasable media is used. Optical media, on the otherhand, is generally not considered erasable because of the manner in which the marks are placed on the media by the laser beam. That is, once a hole or pit or other mark is burned or ablated on the media by the laser beam, it is difficult to remove that hole or pit or mark. However, over time, the media may "flow" or other changes may occur thereto so that the hole, pit, or other mark is somehow altered to the degree that light reflected therefrom might be incorrectly sensed.
A further challenge facing the user of large information systems is the manner in which the data stored is accessed. Certain types of data need only be accessed occasionally, and therefore the access time thereto is not critical. Other types of data are constantly on demand, and therefore must be accessed very rapidly if the system is to operate efficiently. Data bases that are only accessed sequentially or that only need to be accessed occasionally have been traditionally stored on magnetic tape. Data bases that must be accessed quickly, and usually in a random fashion, are stored on magnetic disks. (Access times are significantly faster with magnetic disks because the read/write head of the disk can radially move with respect to any area of the disk and quickly locate a data set within one or two revolutions of the disk.)
An important factor in determining how fast a given data set can be accessed is the manner in which the data is formatted. Coupled with formatting the data is the need to properly index the same so that a desired set of data can be quickly located. The size of the index needed unfortunately grows as the amount of data stored increases. In magnetic-disk art, this has generally not been a major problem because all the magnetic disks are on-line at all times. Thus, one entire disk surface, or even several disks, can be dedicated to indexing information. However, in optical storage systems, it is desirable to have the optical disk removable from the disk drive, much as a record is removed from a phonograph. In this way, only enough drives required to access the data that is continually needed will have to be coupled to the host computer or CPU. Other data, less commonly used, may be stored on a disk and the disk may be physically removed and stored in a suitable location, just as magnetic tape is now removed and stored. Hence, by providing removable optical media, the advantages of both magnetic tape and disk storage systems may be realized. However, when such removable media is used, extreme care must be exercised in defining the formatting and indexing functions so as to preserve most of the data storage space for the storage of user data, not indexing and housekeeping data.
It is thus apparent that there is a need in the art for an optical storage system that not only meets the data capacity and density needs of the exploding data processing industry, but that is also compatible with existing and future high performance CPU systems. Preferably, such an optical storage system will supplement (rather than replace) existing magnetic-based storage systems. That is, a few magnetic disk drives and a few optical disk drives coupled to a main CPU should be able to handle all the existing and future data storage needs of the high information user, instead of the roomsful of magnetic disk and tape drives that such a user must now have installed. However, it is also apparent that there is a need in the art for an optical storage system that provides acceptable data bit error rates, at least on the order of no more than one bit error for every 10E+12 bits of information. Also, the overall data access times must be compatible with the high speed, high performance computers that are presently available. The optical storage system herein disclosed is directed to satisfying these and other needs.